[libc][NFC] Move thread platform data pointer to thread attributes.

Along the way, added constexpr constructors to the Thread data
structures.

libc/include/llvm-libc-types/__thread_type.h

@@ -11,7 +11,6 @@
 
 typedef struct {
   void *__attrib;
-  void *__platform_attrib;
 } __thread_type;
 
 #endif // __LLVM_LIBC_TYPES_THREAD_TYPE_H__

libc/src/__support/threads/linux/thread.cpp

@@ -99,7 +99,8 @@ __attribute__((always_inline)) inline uintptr_t get_start_args_addr() {
 #endif
 }
 
-static void start_thread() __attribute__((noinline)) {
+__attribute__((noinline))
+static void start_thread() {
   auto *start_args = reinterpret_cast<StartArgs *>(get_start_args_addr());
   auto *attrib = start_args->thread_attrib;
   long retval;
@@ -174,7 +175,7 @@ int Thread::run(ThreadStyle style, ThreadRunner runner, void *arg, void *stack,
   auto clear_tid = reinterpret_cast<cpp::Atomic<FutexWordType> *>(
       adjusted_stack + sizeof(StartArgs) + sizeof(ThreadAttributes));
   clear_tid->val = CLEAR_TID_VALUE;
-  platform_data = clear_tid;
+  attrib->platform_data = clear_tid;
 
   // The clone syscall takes arguments in an architecture specific order.
   // Also, we want the result of the syscall to be in a register as the child
@@ -252,7 +253,7 @@ void Thread::wait() {
   // If not, it is a spurious wake and we should continue to wait on
   // the futex.
   auto *clear_tid =
-      reinterpret_cast<cpp::Atomic<FutexWordType> *>(platform_data);
+      reinterpret_cast<cpp::Atomic<FutexWordType> *>(attrib->platform_data);
   while (clear_tid->load() != 0) {
     // We cannot do a FUTEX_WAIT_PRIVATE here as the kernel does a
     // FUTEX_WAKE and not a FUTEX_WAKE_PRIVATE.

libc/src/__support/threads/thread.h

@@ -28,6 +28,7 @@ union ThreadRunner {
 union ThreadReturnValue {
   void *posix_retval;
   int stdc_retval;
+  constexpr ThreadReturnValue() : posix_retval(nullptr) {}
 };
 
 #if (defined(LLVM_LIBC_ARCH_AARCH64) || defined(LLVM_LIBC_ARCH_X86_64))
@@ -87,13 +88,21 @@ struct alignas(STACK_ALIGNMENT) ThreadAttributes {
   int tid;
   ThreadStyle style;
   ThreadReturnValue retval;
+  void *platform_data;
+
+  constexpr ThreadAttributes()
+      : detach_state(uint32_t(DetachState::DETACHED)), stack(nullptr),
+        tls(nullptr), stack_size(0), owned_stack(false), tid(-1),
+        style(ThreadStyle::POSIX), retval(),
+        platform_data(nullptr) {
+  }
 };
 
 struct Thread {
   ThreadAttributes *attrib;
-  void *platform_data;
 
-  Thread() = default;
+  constexpr Thread() : attrib(nullptr) {}
+  constexpr Thread(ThreadAttributes *attr) : attrib(attr) {}
 
   int run(ThreadRunnerPosix *func, void *arg, void *stack, size_t size,
           bool detached = false) {